Field of the Invention
This invention relates to an optical scanning system, which makes an optical scanning in at least one dimension and which includes a multi-faceted reflective rotor. The invention is applicable both to line or one-dimensional scanning and to simultaneous horizontal and vertical or two-dimensional scanning, such as for picture generation.
In use of the invention, a scene or object is rapidly scanned along a line by a scanning system including a rotor positioned in the path of the scanning beam of radiation and having a plurality of contiguous reflective facets arranged as a polygon around the periphery of the rotor. The scanning beam of radiation from the sensed area of the scene is directed through the scanning system to one or more radiation detectors. During each scan the scene is progressively sensed along a horizontal line and the output from the radiation detector therefore is a video signal suitable for representation on a CRT-monitor. For each change of facet in the beam path the scan is repeated. This provides horizontal scanning (scanning in a first dimension). As is well known in the art, simultaneous vertical scanning (scanning in a second dimension) may be effected by an oscillating mirror or some other type of scanning element turning at a lower rate than the polygonal rotor and located at a substantially stationary pupil.
However, this scanning system is suitable not only for radiation detecting purposes but also for picture generation. In the case of picture generation, a rapidly changeable, modulated light source, such as a light emitting diode (LED) or a laser, takes the place of the radiation detector in a scanning system for radiation detection, and the beam path through the scanning system is reversed with respect to the beam path of a radiation detection system.
Also, it is within the scope of the invention to use the same rotor simultaneously for radiation detection purposes and for picture generation purposes. In this case it is possible to use the same optical components and the same multi-faceted rotor for both purposes by inserting beam splitting devices at the input and the output of the scanning system. It is also possible to use different facets on the same rotor but to double the rest of the optical components necessary for the two purposes. In both cases the signal from the radiation detector is used for modulating the light intensity of the rapidly changeable light source.
In the optical scanning systems having a scanning element in the form of a multi-faceted reflective rotor the change-over from one facet to another causes radiation coming from two directions to impinge onto the detector simultaneously, or, in the case of picture generation, two beams to impinge on different portions of the picture simultaneously. In the past, in order to keep the rotor reasonably small the number of facets around the periphery of the rotor therefore had to be restricted to produce an acceptable scanning efficiency, i.e., to produce an acceptable ratio of the length of the part of the scan giving a useful representation of the object to the total length of the scan.
A predetermined horizontal scan frequency is often desired in order that the scanning may be TV-compatible. The greater the number of the facets around the periphery of the rotor can be chosen, the lower the speed of the rotor can be set. Low rotor speed means reduced power consumption and increased service life. Therefore, efforts have been made to increase the scanning efficiency and at the same time increase the number of facets.
One example of such a system is shown in U.S. Pat. No. 4,030,806. In this system, radiation from a laser passes a first positive lens to be focused on the multifaceted rotor. Because the facets are plane, focusing exactly on the facets can only be accomplished at two angular positions of the rotor for each facet, preferably chosen such that the exact focusing occurs when the beam is reflected by a facet near the transitions between two facets. A second positive lens is disposed in the beam path after the area of reflection by the facet to collimate the beam. Plane mirrors disposed outside the rotor then guide the beam onto the rotor a second time and in such a way that the beam is moved in synchronism with the rotor. Although the beam directed to the rotor is focused exactly on the facet only for two angular rotor positions, focus of the beam is very close to the facet throughout the scan, every little irregularity or scratch in the facet will greatly affect the path of the reflected beam. Moreover, the reflected beam takes varying paths between the two positive lenses so that the collimation of the reflected beam is exact only for two angular positions of the rotor for each facet. It has been suggested to eliminate this problem by disposing spherical or cylindrical reflectors outside the rotor to direct the beam onto the rotor the second time. A device of this kind is shown in German published patent application No. DE-A-3 022 365. This device includes a rotor having two axially contiguous sets of facets around the rotor, one set of which has convexely curved facets. Since the beam is focused on the curved facet surfaces of the rotor, scratches and irregularities greatly affect the beam path. Besides, the manufacture of a rotor having such facets is difficult and therefore expensive. The most troublesome problem connected with the manufacture of a rotor having convexely curved reflective facets is the checking measurement to which each manufactured rotor has to be subjected. This checking measurement is practically impossible to make without the aid of special equipment because the transitions between the facets are not as well defined as in the case of plane facets. It is to be noted that even a very small variation of the back focal length of the facets affects the scanning result and that, therefore, the rotor must have very small manufacturing tolerances.